1. Field of the Invention
The present invention relates generally to a cordierite ceramic body, and a process for producing the same, and more particularly to a process for stable or consistent fabrication of a cordierite ceramic article having a low coefficient of thermal expansion, in particular, a ceramic honeycomb structure suitably used as a device for purifying exhaust gases emitted by automotive vehicle engines. The invention is also concerned with a technique for evaluating cordierite compositions for such ceramic bodies, to check if the compositions are suitable for producing such a ceramic article that exhibits a low thermal expansion coefficient.
2. Discussion of the Related Art
There is known a cordierite ceramic article which is produced by firing a formed body of a cordierite batch composition which consists of a mixture including a magnesia source material, a silica source material and an alumina source material, for example, a mixture consisting of talc, kaolin and alumina. Such a cordierite ceramic article or product is known as having high thermal resistance to heat, and having a relatively low thermal expansion coefficient over a wide range of temperatures. For these properties, formed bodies of cordierite ceramics are particularly effectively used as honeycomb catalyst supports used in an apparatus or system for removing hydrocarbons, carbon monoxide, nitrogen oxides, etc., that are contained in various kinds of exhaust gases. Such honeycomb catalyst supports when used for purifying exhaust gases emitted by automobile engines, are subject to high thermal stresses. If the thermal expansion coefficient of the honeycomb structures is higher than 1.1.times.10.sup.-6 /.degree. C. (at 40.degree.-800.degree. C.), these structures are not able to withstand thermal stresses applied thereto during their service, and are likely to be damaged or broken. Therefore, it is required that their thermal expansion coefficient be less than 1.0.times.10.sup.-6 /.degree. C. (at 40.degree.-800.degree. C.: this temperature range being hereinafter applied to any thermal expansion coefficient values), preferably less than 0.8.times.10.sup.-6 /.degree. C.
To satisfy the above requirements, techniques for lowering the thermal expansion coefficient of cordierite ceramic articles have been proposed. For example, U.S. Pat. No. 3,885,977 (published in 1975) proposes a technique of crystal orientation of kaolin as one of components of a cordierite batch composition. Further, U.S. Pat. No. 4,280,845 (published in 1978) proposes a technique of maintaining the particle size of a magnesia source such as talc used as a component of a cordierite composition, within a specific range. There have been also disclosed techniques for reducing the thermal expansion coefficient of cordierite ceramic bodies by using suitable chemical composition of silica, alumina and magnesia, or by establishing suitable conditions in which the cordierite batch compositions are fired into the desired ceramic bodies.
Since a cordierite batch composition for a cordierite ceramic body or article is formed of a mixture consisting of various natural materials such as talc and kaolin, an adjustment of the proportions of the components or the particle size control for each production batch of the cordierite composition, is not effective enough to lower the thermal expansion coefficient of a ceramic article obtained from the thus adjusted or controlled cordierite batch composition. Namely, the obtained cordierite article may have an unexpectedly high coefficient of thermal expansion, depending upon the specific conditions in which the cordierite batch composition is prepared. Thus, it has been considerably difficult to achieve consistently stable fabrication of cordierite ceramic bodies or articles that have a sufficiently low coefficient of thermal expansion.
In the production of cordierite ceramic bodies, some portion of a batch of cordierite composition is rejected for some reason or other, before firing step. For economical production or reduced material cost of the cordierite ceramic products, these rejects in the form of a dried, unfired formed body or segments thereof, are reclaimed as a reclaimed cordierite composition for the same or other cordierite products. However, it is recognized that the cordierite ceramic bodies conventionally formed of such reclaimed cordierite compositions tend to have a comparatively large fluctuation in their coefficient of thermal expansion, and that such reclaimed cordierite compositions are hardly considered to be satisfactory starting materials for producing cordierite ceramic products having a low thermal expansion property. Consequently, the dried, unfired bodies of rejects of a cordierite composition are crushed into a powder form, and used as second-grade materials for fabricating cordierite products that are not required to have a low coefficient of thermal expansion. This fact indicates that a relatively high cost of materials in the production of first-grade cordierite ceramic bodies only have a satisfactorily low coefficient of thermal expansion.
As described above, the reclamation of unfired formed rejects of a cordierite material is considered difficult, particularly for producing a catalyst support structure for purifying automobile exhaust gases, which is required to have a thermal expansion coefficient not exceeding 1.0.times.10.sup.-6 /.degree. C. So far, there has been proposed no effective manner of adjusting such reclaimed cordierite materials, to assure stable production of cordierite ceramic bodies that exhibit a sufficiently low coefficient of thermal expansion.